This invention relates to a high speed spatial light switch, and particularly to a light switch having a short internal light path due to a light input port and a light reflection port being at critical angles close to normal to a switching boundary plane, and wherein operation is based on refractive index changes of electrooptic materials, for example semiconductors incorporating quantum well films, rods or dots.
Optical communication systems having bandwidths of about 10 Gigabits/second (Gb/sec) have been implemented on several thousand kilometer long transoceanic routes, and in the future are expected to operate at about 100 Gb/sec. Switches and modulators are important devices needed in such optical high-speed communication systems. Such devices must be capable of high extinction ratio performance, be small in size, monolithically integrable into integrated circuits and components, fast, have low drive requirements, low loss and be reliable. At present, 10 Gb/sec optical modulators are available, and 40 Gb/sec modulators will soon be or already are available to the market. However, high speed spatial switches that operate in the 10 Gb/sec range do not currently exist, with this switching function currently being performed at slower speeds electronically. Such switching operations are tedious, and two domain transforms (from optical to electronics and from electronics back to optical) are required. There is, therefore, a need for optical spatial switches capable of performing at speeds (in the 100 Gb/sec range) used in newly emerging high speed optical communication systems.
Optical spatial switching can be realized in directional couplers, intersecting (or X type) and branching (or Y type) waveguide structures. In the case of directional couplers, currently available devices are 1 mm or even larger in size, and fabrication accuracy is stringent (1). In the intersecting waveguides where the switching mechanism is based on Total Internal Reflection (TIR), small size and monolithically integrated devices may be realized using semiconductor-type electrooptic materials where refractive index changes of quantum well structures are 1 to 2 orders of magnitude larger than those in bulk materials (2, 3). One problem with these TIR-type switches is poor extinction ratios due to light leakage between ON/OFF states as a consequence of a small angle of intersection (FIGS. 8, 9). The structure of TIR-type switches has been improved by invention of curved and bow-tie electrodes, resulting in better extinction ratios and widened angles of intersection (4, 5, 6, 7). Although such improvements have been experimentally confirmed, there are still problems of high absorption losses caused by unavoidable absorption-increase associated with refractive index changes and necessity for light incidence at grazing angles (8). Since angle of intersection is determined by critical angle, the grazing angle of incidence results in a long light path in the semiconductor switch under voltage-induced absorption losses. The above-mentioned difficulties are serious impediments to realization of high quality optical spatial switches.
In accordance with the foregoing, objects of the invention are directed to overcoming the above-mentioned difficulties. Accordingly, light switching is accomplished in the instant invention by total reflection and transmission of guided modes at a boundary plane of two media of refractive indices n1(t) and n2. In the case of a very high-speed switch, n1(t) is chosen to be a refractive index of a multiquantum well semiconductor material that can be changed very rapidly responsive to application of or removal of an electric field. Therefore, switches of the instant invention comprise optical waveguides fabricated of compound semiconductor materials (e.g. GaInAs/InP for operation at 1.55 xcexcm wavelength or PbTe for operation at 3.39 xcexcm wavelength). Waveguide claddings are fabricated of n- and p-doped semiconductors, and i-type quantum wells are separated by semiconductor barriers. The semiconductor material n1(t) is adjacent to the material of refractive index n2 (air for example), with its boundary plane curved to form an exponential spiral (FIGS. 1(a), 1(b)). Port of incidence and reflection waveguides are positioned at angles equal to or very nearly equal to a critical or Brewster angle for decreasing or increasing refractive index, respectively, as a result of voltage application. A port of transmission waveguide 3 (FIG. 1(b)) may, for example, be an optical fiber, and positioned at the Brewster angle in material of n2 to collect light transmitted into n2.
Electrodes across which a voltage is applied to the semiconductor material n/1(t) in order to effect change of the refractive index thereof are constructed having edges configured to the curved semiconductor boundary, i.e. the electrode edges are also curved to form an exponential spiral. These electrodes are provided on top and bottom of the device, and a uniformly applied electric field provides a corresponding uniform change of refractive index of the electrooptic material. Other advantages and objects of the invention will become clear upon a reading of the following specification.